over warm water. Evaporation from the surface of thewarm water easily saturates the cold air, causing fog,which rises from the surface like smoke. It should benoted that the actual process of heating cold air over awarm surface tends to produce instability. The presenceof an inversion above the surface prevents steam fogfrom rising very high; it is usually fairly dense andpersistent.This type of fog forms on clear nights over inlandlakes and rivers in late fall before they freeze. It isprevalent along the Mississippi River and Ohio River atthat time of year. Arctic sea smoke is the name given tosteam fogs in the arctic region. It forms when cold airmoves over a warmer water surface, which is mostoften found in breaks of the surface ice. It may alsooccur over the ocean surface following a cold frontalpassage when the water is approximately 40°F warmerthan the air passing over it.Upslope Fogs.Upslopefogiscausedbyadiabatic cooling of rising air. It is formed when moist,warm air is forced up a slope by the wind. The coolingof the air is almost entirely adiabatic, since there is littleconduction taking place between the air and surface ofthe slope. The air must be stable before it starts itsmotion so that the lifting does not cause convection, orvertical currents, which would dissipate the fog.Some wind speed is needed, of course, to cause theupslope motion. Upslope fog is usually found where theair moves up a gradual slope. This type of fog is deepand requires considerable time to dissipate. The mostcommon fog of this type is called Cheyenne fog and iscaused by the westward flow of air from the MissouriValley, which produces fog on the eastern slope of theRockies.Frontal Fog.Frontal fog is another hazard,which must be added to the list of weather problemsassociated with fronts. The actual fog is due to theevaporation of falling rain and occurs under the frontalsurface in the cold air mass. This additional water vaporgradually saturates the air. Precipitation falls from thelifted warm air through the cold air. Evaporation fromthe rain continues as long as the temperature of theraindrops is higher than the temperature of the air, eventhough the cold air is already saturated. Naturally, theupper regions become saturated first because thetemperature and dew point are lower at the higheraltitude. As the evaporation from the rain continues, alayer of clouds begins to build down from the frontalsurface. Eventually, this cloud layer extends to theground and becomes fog.During the day, there may be enough turbulencecaused by solar heating to keep this cloud off theground. However, after dark, because of dyingconvection currents and the nocturnal cooling of the air,the ceiling drops suddenly. It is this sudden closing inafter dark that makes frontal fog so dangerous.Cold fronts usually move so rapidly and have suchnarrow bands of precipitation and high wind speeds thatcold-front fog is comparatively rare and short lived.warm-front fog, on the other hand, is fairly common.Since warm frontal systems are quite extensive,warm-front fog may cover a wide area. This type fog isalso deep because it extends from the ground to thefrontal surface. The clouds above the frontal surfacealso slow down the dissipating effect of solar heating.5-9AG5f0504WINDWARM OCEANFOG OR LOWSTRATUSCOLD COASTAL WATERCOLD CONTINENTFigure 5-4.Land advection fog caused by an onshore flow over cold coastal water.